The means by which material to be treated is introduced into each of the tanks and the means by which it is transported out of the tank are means by which other liquid can gain unintended access to the interior of the tank and contaminate the processing solution contained in that tank. If the numerous processing tanks, and more particularly the means by which material to be treated is introduced into each of the tanks and the means by which treated material is transported out of the tanks are all at the same level, which, at least as far as photographic material such as film is concerned, is desirable for various reasons such as to minimize crossover distance and thus time spent by the treated material in the atmosphere during transfer of the material from one tank to another, to facilitate heating of the tank in a water bath, and to simplify forming the tanks as a one piece molding, there is a risk that the processing solution in one of the tanks may be contaminated by liquid from another tank because it is possible that liquid from one tank may splash into another.
In particular, contamination of photographic developer solution with another processing solution could result in damage to a film. That can be a problem when processing black and white film, but it is a far greater problem with development of color film because a greater number of different solutions are used in color development with differing degrees of undesirable consequences when one contaminates another. One of those other solutions used in color development is bleach and contamination of the developer solution with bleach would result in the spoiling or even destruction of the latent images on a customer's film. This is one of the worst things that could happen to a customer's film.
Photographic film processing machines, in particular minilab processors, are usually operated in fixed locations by people with little chemical knowledge, Under these circumstances they may not appreciate the importance of keeping the processing tanks, especially the developer tank, free from contamination by other chemicals.
There is a greater risk of such contamination where the processing machine is mounted on a moving platform, say on a ship, an airplane, a train or a lorry as motion of the platform may cause splashing of the chemical solutions in the tanks and that may lead to contamination. It is quite possible that the operator will not notice such contamination occurring or may not realize the problems that may ensue. Even if such contamination were to be noticed, the necessary remedial action would involve considerable waste of time and materials.